1. Field of the Invention
The present invention relates to an image capturing apparatus and a method for controlling an image capturing apparatus.
2. Description of the Related Art
Solid-state image sensing devices including a CCD or a CMOS image sensor, which are used to capture still images or moving images, are known to have a dynamic range (the ratio of the minimum luminance and the maximum luminance between which tones can be distinguished) which is generally narrower than that of the human eye. Therefore, the problem arises that it is difficult to reproduce tones which are actually viewed by the human eye faithfully in a photograph. To address the problem, there is a known technique of composing a plurality of differently exposed images (an overexposed image (i.e., a light image, hereinafter referred to as an “H image”), a correct-exposure image (hereinafter referred to as an “M image”), an underexposed image (i.e., a dark image, hereinafter referred to as an “L image”), etc.) together to obtain an image having a wider dynamic range (hereinafter referred to as “HDR”) than that of standard imaging capturing. Japanese Patent Laid-Open No. 6-141229 describes an example HDR technique.
According to conventional HDR techniques, after the beginning of shooting, a plurality of differently exposed images (e.g., an H image, an M image, and an L image, i.e., a total of three images) are successively captured. Thereafter, the images are composed together, so that HDR can be achieved by a difference in exposure between the H and L images. In such an HDR technique, however, the images to be composed are captured at different timings, and therefore, a problem arises with the composition of the images if a subject is moving.
To solve this problem, there is a known technique of dividing a picture plane into a plurality of regions, and performing image capturing while changing exposure on a region basis to obtain differently exposed images of each region, thereby reducing a difference between timings at which a plurality of images to be composed (see Japanese Patent Laid-Open No. 2011-244309). According to Japanese Patent Laid-Open No. 2011-244309, the exposure time of the image sensing device can be controlled on a row basis so that a light H image is generated from an overexposed row and a dark L image is generated from an underexposed row. The row to be overexposed and the row to be underexposed can be exposed in parallel, and therefore, the difference between timings at which the H and L images are obtained is reduced.
When L and H images are generated according to the technique of Japanese Patent Laid-Open No. 2011-244309 supra, two or more L images can be generated while one H image is generated because the exposure time of an L image is shorter than that of an H image. (Exactly speaking, if the exposure time of an L image is longer than half the exposure time of an H image, the exposure of the second L image is not completed by the time the exposure of the H image is completed. Even in this case, however, the exposure of the second L image is completed soon after the completion of the exposure of the H image. Therefore, roughly speaking, two or more L images can be generated while one H image is generated.) In Japanese Patent Laid-Open No. 2011-244309, for example, two or more L images which are generated while one H image is generated are utilized so that one H image and two or more L images are composed together.
Incidentally, the first and second L images are obtained at different timings, and therefore, there may be a motion of a subject between the first and second L images. At present, however, there is not a proposal for utilization of two or more L images with attention given to that point.